1. Field of the Invention
The present invention relates to melting and casting devices, and in particular to methods and apparatus for melting and casting precious metals in precision molds.
2. Description of the Prior Art
A known problem encountered in connection with fine casting of metals is the influence of the ambient atmosphere on the molten metal. Another problem is that gas pockets may remain entrapped in the mold, tending to inhibit the flow of the melt, especially to remote points of complicated, finely branched molds. An additional limitation to the permissible complexity and cross-sectional configuration of the mold branches is the surface tension of the melt and a tendency of the oxygen of the atmosphere to produce oxidation products with the melted metal or with melt alloy components.
In order to overcome these difficulties, it has already been suggested in the prior art to evacuate the air contained in the mold cavity, or molding cell, and to introduce a rare gas, such as argon, for example, into the crucible. The melt is tapped by opening a valve in the bottom portion of the crucible, whereupon it flows through a sealed connecting channel into the cell. This procedure effectively prevents the formation of oxide films on the casting.
In the case of very finely branched molds, as are encountered, for example, in connection with the casting of jewelry, the above-mentioned problems are particularly severe, representing practical limitations to the complexity of shapes which are castable, because the vacuum inside the molding cell, which latter is pre-heated, tends to induce undesirable evaporation and precipitation of the alloy or of certain alloy components. Obviously, the degree of the difficulties encountered is directly related to the level of the vacuum obtained inside the molding cell. Thus, a tradeoff has to be made between the desire to completely eliminate all gas pockets through the establishment of a high vacuum inside the molding cell and the tendency of partial evaporation and the accompanying alloy precipitation phenomenon.